Fuel cell bipolar plate exit for improved flow distribution and freeze compatibility

ABSTRACT

A fuel cell assembly is disclosed that utilizes a water transport structure extending from fuel cell plates of the assembly into fuel cell assembly manifolds, wherein the water transport structure facilitates the transport of liquid water from the fuel cell plates thereby minimizing the accumulation of liquid water and ice in the fuel cell stack.

FIELD OF THE INVENTION

The invention relates to a fuel cell assembly, and more particularly toa fuel cell assembly utilizing water transport structures partiallydisposed in a manifold of the fuel cell stack to facilitate thetransport of liquid water from the fuel cell assembly.

BACKGROUND OF THE INVENTION

Fuel cell power systems convert a fuel and an oxidant to electricity.One type of fuel cell power system employs a proton exchange membrane(hereinafter “PEM”) to catalytically facilitate reaction of fuels (suchas hydrogen) and oxidants (such as air or oxygen) to generateelectricity. The PEM is a solid polymer electrolyte that facilitatestransfer of protons from the anode to the cathode in each individualfuel cell of a stack of fuel cells normally deployed in a fuel cellpower system.

In a typical fuel cell stack of a fuel cell power system, individualfuel cells provide channels through which various reactants and coolingfluids flow. Fuel cell plates may be unipolar, or a bipolar plate may beformed by combining a plurality of unipolar plates. Fuel cell plates maybe designed with serpentine flow channels. Serpentine flow channels aredesirable as they effectively distribute reactants over the active areaof an operating fuel cell, thereby maximizing performance and stability.Movement of water from the flow channels to outlet manifolds of the fuelcell plates is caused by the flow of the reactants through the fuelcell. Drag forces cause the liquid water to flow through the channelsuntil the liquid water exits the fuel cell through the outlet manifolds.However, when the fuel cell is operating at a lower power output, thevelocity of the gas flow is too low to produce an effective drag forceto transport the liquid water, and the liquid water accumulates in theflow channels.

A further limitation of relying on gas flow drag forces to remove theliquid water is that the drag forces may not be strong enough toeffectively transport the liquid water creating pinning points that maycause the water to accumulate and pool, thereby stopping the water flow.Such pinning points are those commonly located where the channel outletsmeet the fuel cell stack manifold.

Some current fuel cell assemblies utilize plates having hydrophilicsurfaces. Water has been observed to form a film on the surface of thematerial and accumulate at the outlet of the flow channels and theperimeter of the plates. The water film can block the gas flow, which inturn reduces the driving force for removing liquid water and preventsthe removal of the liquid water from the fuel cell stack. Theaccumulation of water can cause gas flow blockages or flow imbalancesthat can have negative impacts on the performance of the stack.

Further, the accumulated water may form ice in the fuel cell assembly.The presence of water and ice may affect the performance of the fuelcell assembly. During typical operation of the fuel cell assembly, wasteheat from the fuel cell reaction heats the assembly and militatesagainst vapor condensation and ice formation in the assembly. During astarting operation or low power operation of the fuel cell assembly insubzero temperatures, the condensed water in the flow channels of thefuel cell plates and at edges of the outlet manifolds may form icewithin the fuel cell assembly. The ice formation may restrict reactantflow, resulting in a voltage loss.

It would be desirable to develop a fuel cell assembly with an improvedmeans for removing liquid water from fuel cell gas flow channels of thefuel cell stack to minimize the accumulation of liquid water and ice inthe fuel cell assembly.

SUMMARY OF THE INVENTION

Concordant and congruous with the present invention, a fuel cellassembly with an improved means for removing liquid water from fuel cellgas flow channels of the fuel cell assembly to minimize the accumulationof liquid water and ice in the fuel cell assembly, has surprisingly beendiscovered.

In one embodiment, the fuel cell plate comprises a plate having a firstaperture formed therein; a plurality of flow channels formed on saidplate; and a water transport structure disposed between at least one ofsaid flow channels and the aperture of said plate to facilitate atransport of water from the at least one of said flow channels to theaperture.

In another embodiment, the fuel cell plate comprises a bipolar plate; aplurality of flow channels formed on each face of said bipolar plate; atleast one aperture formed through said bipolar plate; and a watertransport structure, wherein said water transport structure includes afirst end disposed through an aperture formed in a face of said bipolarplate between the flow channels and the aperture, an intermediateportion disposed between the faces of said bipolar plate, and a secondend extending from the intermediate portion into the aperture.

In another embodiment, the fuel cell assembly comprises a fuel cellstack including a plurality of fuel cell plates, each fuel cell platehaving a plurality of flow channels and a plurality of faces, whereineach fuel cell plate includes at least one aperture formed therein, theapertures of the fuel cell plates substantially aligned to form amanifold; and a water transport structure extending into the manifoldfrom an inner edge of the aperture of each fuel cell plate, whereinwater is caused to flow from the fuel cell plate, through said watertransport structure, and through the manifold.

DESCRIPTION OF THE DRAWINGS

The above, as well as other advantages of the present invention, willbecome readily apparent to those skilled in the art from the followingdetailed description of a preferred embodiment when considered in thelight of the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a fuel cell stack incorporating awater transport structure in a fuel cell stack manifold according to anembodiment of the invention;

FIG. 2 is a schematic cross-sectional view of the water transportstructure of the fuel cell stack illustrated in FIG. 1;

FIG. 3 is a top plan view of a fuel cell plate of the fuel cell stackillustrated in FIG. 1; and

FIG. 4 is an enlarged fragmentary top plan view of the fuel cell plateillustrated in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description and appended drawings describe andillustrate various exemplary embodiments of the invention. Thedescription and drawings serve to enable one skilled in the art to makeand use the invention, and are not intended to limit the scope of theinvention in any manner.

FIGS. 1 and 2 show a fuel cell assembly 10 according to an embodiment ofthe invention. The fuel cell assembly includes a plurality of stackedfuel cell plates 12. Each of the plates 12 includes an inlet aperture,an outlet aperture, and a plurality of water transport structures 18.The inlet apertures of each of the plates 12 cooperate to form an inletmanifold 14 and the outlet apertures of each of the plates 12 cooperateto form an outlet manifold 16. The inlet manifold 14 is in fluidcommunication with an inlet 28 and the outlet manifold 16 is in fluidcommunication with an outlet 30. It is understood that the fuel cellassembly 10 shown in FIGS. 1 and 2 may be a cross-section of either ananode side or a cathode side.

FIGS. 3 and 4 show a top plan view of a bipolar fuel cell plate 12formed from a pair of unipolar plates. The bipolar plate 12 includes twoinlet apertures 20, two outlet apertures 22, and a plurality of flowchannels 24. It is understood that the flow channels 24 may include thechannels disposed on an external face of the fuel cell plate 12, as wellas the passages disposed intermediate internal faces of the fuel cellplate 12. It is also understood that the material of construction, size,shape, quantity, and type of plates 12 in the fuel cell assembly 10, andthe configuration of the fuel cell plates 12 within the assembly 10, mayvary based on design parameters such as the amount of electricity to begenerated, the size of the machine to be powered by the fuel cellassembly 10, the volumetric flow rate of gases through the fuel cellassembly 10, and other similar factors, for example. The fuel cellplates 12 may be formed from any conventional material such as graphite,a carbon composite, or a stamped metal, for example. The fuel cell plate12 shown in FIG. 3 may be used for an anode side or for a cathode side(not shown) of the fuel cell assembly 10. Further, it is understood thatthe plate 12 may have any number of inlet apertures 20 and outletapertures 22, as desired. As shown, the flow channels 24 are undulated.However, the flow channels 24 may be substantially linear, serpentine,or have other configurations, as desired.

Water transport structures 18 are disposed on the fuel cell plate 12 atthe inlet apertures 20 and the outlet apertures 22, as shown in FIGS. 3and 4. It is understood that more or fewer water transport structures 18can be used as desired. The water transport structures 18 include afirst end 18 a, a second end 18 c, and an intermediate portion 18 bformed between the first end 18 a and the second end 18 c.

The first ends 18 a of the water transport structures 18 extend intoapertures 26 formed in the fuel cell plate 12 intermediate the flowchannels 24 and the inlets 20 and intermediate the flow channels 24 andthe outlets 22. Typically, the apertures 26 are formed intermediate agasket 32 and the flow channels 24, as shown in FIG. 2, although otherconfigurations can be used if desired.

The intermediate portions 18 b of the water transport structures 18 aredisposed between the unipolar plates of the fuel cell plate 12. In theembodiment shown, the intermediate portions 18 b of the water transportstructures 18 circumvent the gasket 32. Accordingly, a flow path isprovided adjacent the gasket 32, as shown in FIG. 2.

The second ends 18 c of the water transport structures 18 extend frombetween the fuel cell plates 12 and into the inlet apertures 20 andoutlet apertures 22. In the embodiment shown, the water transportstructures 18 have a substantially rectangular shape. However, the watertransport structures 18 may have any shape as desired such as atriangular shape, a curvilinear shape, and an irregular shape, forexample. As illustrated in FIGS. 1 and 2, the second ends 18 c of thewater transport structures 18 depend downwardly due to gravity, therebycausing adjacent second ends 18 c to substantially abut. However, it isunderstood that the second ends 18 c can hang individually and in otherconfigurations as desired.

The water transport structures 18 may be formed from any non-conductiveporous material such as a foam, cotton, wool, glass fibers, felt,flocked fibers, paper, and paper and polymer fiber composites, forexample. The water transport structure 18 may also include a hydrophiliccoating such as a silicon oxide (SiO_(x)), another metal oxide, or otherchemical coating, for example, a hydrophobic coating, or be formed froma hydrophilic or hydrophobic material.

The inlet manifold 14 includes the inlet 28 in fluid communication withthe inlet manifold 14 formed in the fuel cell assembly 10 by the inletapertures 20 of the fuel cell plates 12. The plates 12 are stacked withthe inlet aperture 20 of each plate 12 substantially aligned with theinlet aperture 20 of an adjacent plate or fuel cell plates 12. It isunderstood that the diameter, quantity, and length of the inlet manifold14 will depend on the size and quantity of inlet apertures 20 in thefuel cell plates 12 and the number of fuel cell plates 12 stacked in thefuel cell assembly 10.

The outlet manifold 16 includes the outlet 30 in fluid communicationwith the outlet manifold 16 formed in the fuel cell assembly 10 by theoutlet apertures 22 of the fuel cell plates 12. The plates 12 arestacked with the outlet aperture 22 of each plate 12 substantiallyaligned with the outlet aperture 22 of an adjacent plate or plates 12.It is understood that the diameter, quantity, and length of the outletmanifold 16 will depend on the size and quantity of outlet apertures 22in the plates 12 and the number of plates 12 stacked together in thefuel cell assembly 10.

Generally, during operation of a fuel cell power system, a hydrogenreactant is fed into the anode side of the fuel cell assembly 10.Concurrently, an oxygen reactant is fed into the cathode side of thefuel cell assembly 10. On the anode side, the hydrogen is catalyticallysplit into protons and electrons. The oxidation half-cell reaction isrepresented by: H₂←→2H⁺+2e⁻. In a polymer electrolyte membrane fuelcell, the protons permeate through the membrane to the cathode side. Theelectrons travel along an external load circuit to the cathode sidecreating the current of electricity of the fuel cell assembly 10. On thecathode side, the oxygen reacts with the protons permeating through themembrane and the electrons from the external circuit to form watermolecules. This reduction half-cell reaction is represented by:4H⁺+4e⁻+O₂←→2H₂O. Anode exhaust from the anode side flows through abackpressure control valve to a combustor, or is alternatively recycledback to the anode inlet manifold. Cathode exhaust from the cathode sideflows through a second backpressure control valve to the combustor or tothe ambient environment. A control module typically regulates theconditions of the hydrogen stream, oxygen stream, and exhaust streams byoperating various control valves, backpressure control valves andcompressors in response to signals from pressure sensors and electricalpower sensors connected to the fuel cell assembly 10.

During operation of the fuel cell assembly 10, droplets of liquid waterare formed in the channels 24 of the fuel cell plates 12 on the cathodesides of the fuel cell assembly 10. Some water also may be transportedinto the anode flow channels, or may form in the anode channels viacondensation resulting from consumption of the hydrogen. It isunderstood that the operation as described herein for the cathode sideis similar to operation for the anode side of the fuel cell assembly 10.The air stream flowing through the cathode side causes the waterdroplets to flow through the channels 24, toward the outlet manifold 16.Water vapor also flows towards the outlet manifold 16. Once the waterdroplets contact the first ends 18 a of the water transport structures18, the water is wicked away from the channels 24 by the water transportstructures 18, through the intermediate portions 18 c, and into themanifolds 14, 16 from the second ends 18 b. Because the apertures 26 areformed intermediate the gasket 32 and the flow channels 24, the waterand vapor may be removed from the assembly 10 while also facilitatingproper sealing by the gasket 32. If the water transport structures 18are spaced apart as shown in FIG. 3, water and water vapor will also betransported past the water transport structures 18 through the manifolds14, 16 and from the fuel cell assembly 10 in the known methods of waterremoval. If the water transport structures 18 include a hydrophiliccoating, or are produced from a hydrophilic material, this will provideadditional capillary force to attract the water droplets and thecondensed water vapor. The exhaust gas streams also pass through thewater transport structures 18, and through the outlet manifold 16. Ifthe water transport structures 18 include a hydrophobic coating or areproduced from a hydrophobic material, capillary action is aided by therepulsive nature of the coating or material.

It is expected that three different water transport mechanisms may beutilized to remove the water from the channels 24, depending on thematerial used for the water transport structures 18. First, the porousmaterials 18 may form a network of open, continuous pores that arecapable of utilizing capillary forces to transport the watertherethrough. Second, because the second ends 18 c of the watertransport structures 18 abut and form continuous paths through the inletmanifold 14 and the outlet manifold 16 of the fuel cell assembly 10, thewater absorbed by the water transport structures 18 will create a staticpressure head to facilitate removal of the water from the manifolds 14,16. It is desirable, though not necessary, for a portion of the watertransport structures 18 to be saturated to create a sufficient head tocause the water to drain from the water transport structures 18. If asaturated portion is not created in the water transport structures 18, aperistaltic pump (not shown) may be used with the fuel cell assembly 10to cause the water to flow through the assembly 10 and out of the watertransport structures 18. The peristaltic pump may be a peristaltic pumpsuch as the one disclosed by Anonymous, Pump to Remove Water from aWick, Pub. No. 494084, O. G. June 2005. Third, during operation of thefuel cell assembly 10, it is anticipated that a portion of anoperational cycle will result in the outlet reactants streams being lessthan saturated, wherein evaporation will aid water removal from thewater transport structures 18.

From the foregoing description, one ordinarily skilled in the art caneasily ascertain the essential characteristics of this invention and,without departing from the spirit and scope thereof, can make variouschanges and modifications to the invention to adapt it to various usagesand conditions.

1. A fuel cell plate comprising: a plate having a first aperture formedtherein; a plurality of flow channels formed on said plate; and a watertransport structure disposed on said plate and in fluid communicationwith at least one of said flow channels and the first aperture of saidplate; said water transport structure comprising a first end, a secondend, and an intermediate portion, the second end formed of a porousmaterial and extending downwardly from said plate and into the firstaperture, said water transport structure configured to wick away waterfrom the at least one of said flow channels through said second end tothe first aperture.
 2. The fuel cell plate of claim 1, wherein saidplate includes a second aperture formed therein between said flowchannels and the first aperture, the first end of said water transportstructure disposed in the second aperture.
 3. The fuel cell plate ofclaim 1, wherein said water transport structure is a porous materialformed from one of a foam, cotton, wool, glass fibers, a felt, flockedfibers, a paper, and a paper and polymer fiber composite.
 4. The fuelcell plate of claim 1, wherein said water transport structure isnon-conductive.
 5. The fuel cell plate of claim 1, wherein said watertransport structure is produced from one of a hydrophilic material and ahydrophobic material.
 6. The fuel cell plate of claim 1, wherein saidwater transport structure includes one of a hydrophilic coating and ahydrophobic coating.
 7. The fuel cell plate of claim 6, wherein thehydrophilic coating is a metal oxide.
 8. The fuel cell plate of claim 7,wherein the hydrophilic coating is silicon oxide.
 9. The fuel cell plateof claim 1, further comprising a plurality of said water transportstructures spaced apart from one another and disposed on said fuel cellplate.